316SS Forging process work flow

[Careful]

Hi all,

We are receiving forged 316SS parts that still have scale on them and, without much knowledge about the forging industry and it’s processes, trying to figure out the best way to specify they be cleaned to pass a B-117 salt spray test.

If you’d indulge me, I some really basic questions:

Does the scale form in preheating process or during the forming process?

If the part is 316SS why does the scale rust? (does it have to do with tool steel dies?)

What’s the typical way of fixing this? (Pickling? Electro-polish?)

At what point in the process would this be done? (Pickling would be after forging and before machining, correct? Then passivation after machining?)

Does annealing play any part in cleaning process, or is it only for adjusting hardness?

Thanks very much in advance!

 

[AndrewFinAustralia]

Hi Careful,

There's a lot here that work with/specify stainless daily - I'll defer to them.

To answer the question "Why does the scale rust" - Scale is, by definition, the oxide layer. Either this oxidised stainless has picked up chloride/sulfur compounds during fabrication, and you're seeing ferrous chloride, or the Nickel and Chromium are burnt out of the scale to a point where the remnant NickelChromium [edit] is below 12% and the scale does not have the contiguous oxide layer present on the surface any more that makes stainless steel "stainless" and the iron present oxidises.

AF

 

[EdStainless]

You could suppress the scale formation in preheat by using a protective atmosphere, but some oxide scale would form during the forging.
Some parts are allowed to be 'process annealed', that is force cooled from a temperature above the minimum annealing temp.
Some parts (largely depending on specification) require a separate reheat for annealing.
Likewise the final anneal could be done in hydrogen for a bright anneal. If this is done then the scale is removed before the BA.
Typically the scale would only be removed once. So if pre-heat, forge, anneal (in air), then grit blast and pickle. And follow any grinding or machining with passivation for good clean surfaces.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

Are the parts hot forged or cold forged? The typical recommendation for hot forging 316 cres is at temperatures above 1700degF.

It is possible that the forging dies or trimming tools may be contaminating the part surface. Passivation of the part surface (AMS 2700 or similar) should remove any ferrous contaminants, and should be performed after finish machining. There should be no problem passing ASTM B117 testing.

 

[Careful]

They are being hot forged. We thought passivation would be sufficient too, but it is turning out to be very much not in this case, for some reason.

 

[AndrewFinAustralia]

Careful,

Just to add (and I'm aware that I'm probably telling you something you know already) the scale is ceramic and abrasive - it can pick up small particles of material from the die.

AF

 

[EdStainless]

Passivation alone is not enough, you need to remove the layer of Cr depleted material that was under the scale, as well as particles of embedded scale in the surface.
Blast first to break up the scale, then pickle to clean them.

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P.E. Metallurgy, Plymouth Tube